Retail gasoline transactions represent a compromise between customer convenience and environmental as well as safety concerns. From the customer""s perspective, any extension of time required to complete the fuel dispensing transaction represents an inconvenience. While from the perspective of governmental regulators and other concerned groups, excessive fuel dispensing rates represent an environmental and safety hazard. There are a number of valid reasons underlying these concerns.
For example, cars sold in the United States after 1998 are required to have onboard refueling vapor recovery (ORVR) systems to minimize vaporous emissions during vehicle refueling operations. At excessive refueling flow rates, such systems are overwhelmed and fail to function effectively. Moreover, excessive fueling rates result in xe2x80x9cspit back,xe2x80x9d wherein a portion of liquid fuel splashes out from the vehicle""s fueling port as the customer xe2x80x9ctops offxe2x80x9d the fuel tank at the conclusion of refueling. Spilling liquid fuel poses obvious environmental and safety concerns. In response, 40 C.F.R. 80.22, issued by the Environmental Protection Agency (EPA), provides rules governing the refueling of motor vehicles. In essence, these rules limit fuel-dispensing rates to no more than 10 gallons per minute (GPM) from any fuel dispenser not exclusively dedicated to heavy-duty vehicles, boats, or airplanes.
While subject to much variation, a basic fuel dispensing system includes a fuel source (e.g., an underground storage tank), a pump in fluid communication with the underground storage tank (e.g., a submersible turbine pump), and at least one fuel dispenser connected to the pump via a network of fluid conduits. When the fuel dispenser is activated, the pump begins pumping fuel from the tank to the fuel dispenser at a given pressure. Normally, the pump is configured to deliver fuel at a pressure and flow rate that allows the fuel dispenser to dispense fuel at a desired or nominal fuel discharge rate. Typically, the fuel dispenser includes a flow sensor (e.g., flow meter) allowing it to monitor the rate of fuel dispensed from its fuel discharge outlet, and a flow control device (e.g., a control valve) allowing it to adjust the discharge rate.
Based on monitoring the flow sensor, the fuel dispenser adjusts its flow control device to maintain the nominal fuel discharge rate, such as the mandated 10 GPM limit. In a multiple fuel dispenser installation, the pump is typically sized and configured to provide fuel at a pressure adequate to ensure that the fuel dispenser having the greatest pressure drop with respect to the pump has adequate pressure and flow to deliver fuel at the nominal fuel discharge rate. Usually, the worst-case fuel dispenser is furthest from the pump/tank, and therefore experiences the greatest conduit-related pressure loss.
This basic approach has aspects of simplicity, but includes obvious drawbacks. For example, while insuring that any individually active fuel dispenser provides fuel at the nominal fuel discharge rate, it does not readily accommodate situations where multiple dispensers are simultaneously in use. One workaround to this problem is to simply size and configure the pump (or pumps) to operate by default at a pressure high enough to provide a selected number of simultaneously active fuel dispensers with adequate flow to ensure that each one provides the requisite nominal fuel discharge rate. Drawbacks to this approach include the inefficiency of operating the fuel pump(s) at a greater pressure than required for dispensing transactions that do not require the maximum pump output, and the greater pump wear incurred at higher operating pressures. Moreover, regulations limit operating pressure of the pump, so this approach has practical as well as regulatory limits.
Some fuel dispensing systems make no real provisions for the problems associated with multiple active fuel dispensers. Such installations are a source of frustration for busy customers, as the actual fuel-dispensing rate from each active fuel dispenser decreases with each newly activated fuel dispenser. During peak refueling times, such fuel dispensing systems operate so slowly that the effective number of fueling transactions per hour is significantly reduced, thereby diminishing the profitability of the fueling station. Additionally, the dissatisfaction of consumers subjected to interminable refueling times may be such that they avoid further patronage of the offending fueling station, resulting in a long-term loss of repeat business.
Constant pressure pumps stand as an alternative to the above-described systems. Constant pressure pumps use their pump outlet pressure as a control variable. Systems of this type vary pump speed such that pump outlet pressure remains essentially constant across a range of flow rates. Thus, as additional fuel dispensers are activated, the constant pressure pump responds by increasing its pumping rate in an effort to provide each active fuel dispenser with adequate flow. While representing an improvement over fixed- or single-speed pump installations, constant pressure pumps add cost and complexity to the system. Further, because pump control is based on outlet pressure sensing rather than actual dispenser discharge rates, constant pressure pumps do not guarantee that each active fuel dispenser actually achieves its nominal fuel delivery rate.
As such, there remains a need for a fuel dispensing system that operates in a manner that maintains a nominal fuel discharge rate from each fuel dispenser regardless of the number of active fuel dispensers, up to reasonable design limits. Ideally, the system would vary the pumping characteristics of the fuel pumps providing fuel to the active fuel dispensers based on monitoring the fuel discharge rate from each active fuel dispenser, thereby ensuring that each dispenser actually delivers fuel at the nominal fuel discharge rate. Further, such monitoring ideally utilizes existing dispenser hardware, thereby minimizing the incremental cost of the improved fuel dispensing system while simultaneously easing the complexity of retrofitting existing fuel dispensing systems.
The present invention provides both methods and apparatus allowing a fuel dispensing system to adjust the pumping rate of fuel supplied to one or more fuel dispensers, based on dispenser feedback signals. Each fuel dispenser includes a flow control device for controlling the flow rate and a flow sensor for measuring the actual fuel discharge rate. The dispenser adjusts its flow control device to achieve a nominal fuel discharge rate, based on monitoring the actual fuel discharge rate from its fuel discharge outlet. Additionally, the fuel dispenser provides a dispenser feedback signal that may be used by the fuel dispensing system to vary the pumping rate/pressure of fuel supplied to the fuel dispenser.
Nominally, each fuel dispenser controls its fuel discharge rate via its flow control device. However, under certain conditions, the current pumping rate from the fuel source supplying the fuel dispenser may be inadequate to allow the fuel dispenser to achieve its nominal fuel discharge rate. In this case, the fuel dispenser may use its feedback signal to indicate the low flow rate condition. In response to this indication, a pump controller comprising a portion of the fuel dispensing system may increase its pumping rate such that all active fuel dispensers operating from the affected fuel source(s) achieve their nominal fuel discharge rates.
When multiple fuel dispensers are simultaneously active, each one may experience varying degrees of low flow rate. The pumping rate is adjusted within design limits to permit the worst-case low flow rate dispenser to achieve the nominal fuel discharge rate. In concert, any active fuel dispenser that would otherwise dispense fuel at greater than the nominal fuel discharge rate because of the increased pumping rate, adjusts its flow control device to maintain the nominal fuel discharge rate. Thus, the system of the present invention provides fuel source pumping control based on actual fuel discharge rates as monitored at each active fuel dispenser. Of course pumping rate control may also include consideration of other inputs to provide more effective rate control. For example, dispenser error signals may be used to prevent pumping control based on signals from malfunctioning fuel dispensers.
In one embodiment, each fuel dispenser in the fuel dispensing system provides a discrete fuel dispenser feedback signal. During dispensing operations, an active fuel dispenser adjusts its flow control device for a given fuel source pumping rate to achieve a nominal fuel discharge rate. Whenever the fuel discharge rate falls below the nominal discharge rate and appropriate flow control device adjustments fail to remedy the low flow rate condition, the fuel dispenser asserts its fuel dispenser feedback signal.
A pump interface system receives these discrete signals and increases the fuel-pumping rate up to a maximum rate whenever at least one dispenser feedback signal is asserted. Pumping rate adjustments may be made in stepwise fashion to provide a damped control response. For example, when a dispenser feedback signal is asserted, the pumping system may increase its pumping rate by a determined amount. After an appropriate settling time, the pumping system may determine whether a fuel dispenser feedback signal is still asserted and, if so, increase its pumping rate again, and so on. In this control scenario, the pumping rate would be maintained at the rate which caused all dispenser feedback signals to be de-asserted. An alternative scheme would be to simply maintain this pumping rate until there are no active fuel dispensers, whereupon the pumping rate logic and nominal pumping rate could be reset in anticipation of the next series of fueling transactions.
Another embodiment provides feedback signals from the fuel dispensers representative of the actual flow rate of the dispenser. A controller or intelligent logic receives such feedback signals so that such controller can ascertain the flow rates of the dispensers with respect to the desired flow rate to determine best how to control the pumping rate to achieve maximum flow rate up to design limits.
Providing feedback signals from the fuel dispensers proportionate with actual fuel dispenser discharge rates allows more sophisticated pump control. Such proportionate signals may be used by the pump control system to precisely adjust the pumping rate to a level that allows all active fuel dispensers to achieve their nominal fuel discharge rate. This allows the pump control system to pump at a rate (or pressure) no greater than that required by the particulars of the current dispensing scenario. These particulars include the number of concurrently active fuel dispensers and the pressure drops associated with the fluid conduits between the remote pump(s) and the active fuel dispensers.
Thus, with proportionate dispenser feedback control, the fuel dispensing system of the present invention can tailor pumping operations over a range of fuel dispensing scenarios. For example, the pump control system may operate the pump(s) at a different rate or pressure for an active dispenser that is close to the pump(s), as compared to the operating rate or pressure for an active dispenser that is far removed from the pump(s). In this manner, the energy consumed by the fuel pumping system is minimized, as is unnecessary wear on the pumps themselves.